The present invention relates to stretch wrap film, and its use in the palletisation of loads. In particular, the invention relates to blown stretch wrap film which generates low noise in use.
When using a stretch film to wrap goods, either using a hand applicator or a semi- or totally automated machine, considerable noise may be generated as the stretch film is unwound from its supply roll. In some cases, the noise generated may be in excess of that which can be safely tolerated for long term exposure by the human ear. This is particularly disadvantageous when the film is to be unwound by hand or using a semi-automated machine, because of the presence of personnel close to the supply roll.
There have been various proposals to overcome this problem. One such proposal has been to employ a manufacturing method known as chill roll casting. This method involves extrusion of a polymer melt on to water-cooled rollers, thereby achieving rapid cooling of the resultant film. However, it has not proved possible to adapt this technology to the manufacture of the traditionally tougher, stiffer and more crystalline blown films.
Another proposal to overcome the problem of excess unwind noise is disclosed in the Applicant""s co-pending patent application WO-A-9629203, in the form of a one-sided cling film having, on one side of the film, a cling layer comprising ultra low density polyethylene (ULDPE), and, on the other side of the film, a non-cling layer comprising low density polyethylene (LDPE) and a solid particulate antiblock additive. The noise levels reported for films of this type range from 85 dB to 96 dB, and therefore except for those films generating a noise level on unwinding at the lower end of this range, it remains necessary, for hearing safety, to limit the period for which an operator is exposed to unwinding of the film.
Furthermore, a problem associated with the use of antiblock additives is that these may be present at the surface of the film during film manufacture and usage, causing undesirable build-up on equipment, necessitating stoppage for cleaning, and diluting the desired effects of those additives in the final film. Some antiblock additives also weaken the film, leading to premature film fracture, and may also detract from film clarity.
WO-A-9515851 discloses blown and cast one-sided cling stretch wrap films which do not require the use of additives or functional polymers to improve their cling properties. These films comprise a cling layer of an homogeneously-branched ethylene polymer having a density of up to about 0.90 g/cm3, and a non-cling polyolefin layer having a density of greater than about 0.90 g/cm3. However, because WO-A-951581 is not concerned with the production of blown stretch wrap films which, on unwinding from a supply roll, generate comparatively low noise, it does not disclose or give any indication of the noise level generated on unwinding its films.
According to a first aspect of the present invention, a multilayer blown stretch film comprises a first layer having cling properties and comprising a copolymer of ethylene and a C4-8 xcex1-olefin, the copolymer having a density in the range 0.850 to 0.890 g/cm3, and a second layer having substantially no cling properties and comprising at least 70 wt. % of a low density polyethylene having a density in the range 0.915 to 0.940 g/cm3.
The noise level generated during unwinding of this film is a maximum of 85 dB, and is typically significantly less than this, for instance around 82 dB or less. Accordingly the period for which an operator may be exposed to unwinding of the film is not limited by hearing safety.
According to a second aspect of the present invention, a method of making a multilayer blown stretch film comprises co-extruding a copolymer of ethylene and a C4-8 xcex1-olefin, the copolymer having a density in the range 0.850 to 0.890 g/cm3, with a polymer composition comprising at least 70 wt. % of a low density polyethylene having a density in the range 0.915 to 0.940 g/cm3, to form a tube having the ethylene copolymer in one layer and the polymer composition in another layer, and blowing and then cooling the tube, by blown film co-extrusion techniques known to those skilled in the art.
According to a third aspect of the present invention, a film as described above is used in the palletisation of loads.
The multilayer film of the present invention is a stretch film, by which we mean it may be stretched by more than 100%, and typically by more than 200%, of its original, unstretched, length, without damage thereto or loss of one-sided cling properties.
The film may be an A/B film, in which layer A is the cling layer and layer B the non-cling layer. Preferably, the film may include one or more layers in addition, and positioned between, the cling and non-cling layers, to add bulk or to confer other properties on the film. In this case, the properties of the intermediate, or core, layer or layers are typically selected according to the application for which the film is ultimately intended.
The cling layer of the film comprises a copolymer of ethylene and a C4-8 xcex1-olefin, and has a density in the range 0.850 to 0.890 g/cm3, preferably in the range 0.860 to 0.885 g/cm3, and more preferably in the range 0.865 to 0.875 g/cm3. Suitable ethylene copolymers are very low density polyethylenes (VLDPE), ultra low density polyethylenes (ULDPE), or linear low density polyethylenes (LLDPE), referred to in the industry as xe2x80x9cpolyolefin plastomersxe2x80x9d, abbreviated to xe2x80x9cPOPsxe2x80x9d. Such materials are amorphous in nature. Commercial examples include TAFMER (Mitsui Chemical), AFFINITY and ENGAGE (Dow Chemical), DEX PLASTOMERS (DSM/Exxon JV) and EXACT (Exxon Chemical). Homogeneously-branched copolymers are preferred, and in particular homogeneously-branched POPs.
The term xe2x80x9chomogeneously-branchedxe2x80x9d in the context of ethylene polymers is well understood in the field, and is defined, for instance, in WO-A-9515851. In summary, it is used to define copolymers of ethylene and an xcex1-olefin, for instance butene, hexene or octene, in which the xcex1-olefin monomer is randomly distributed within a given molecule; substantially all the copolymer molecules have the same ethylene: xcex1-olefin monomer ratio; the copolymer has a narrow short chain (i.e. less than 6 carbon atoms) branching distribution; and the copolymer essentially lacks a measurable high density fraction.
The ethylene copolymer for use in the invention may be manufactured by any of the known processes. For instance, the preferred homogeneously-branched ethylene copolymer may be manufactured as described in WO-A-9515851, or the various patents referred to therein.
Preferably, the ethylene copolymer has a melt index in the range 0.5 to 3.0 g/10 min, preferably 0.5 to 1.5 g/10 min, as measured by ASTM norm D1238 (190xc2x0 C./2.16 kg), for ease of processing.
The polymer composition for use as the non-cling layer comprises at least 70 wt. %, based on the weight of the non-cling layer, of a conventional long chain (i.e. greater than 6 carbon atoms) branched low density polyethylene (LDPE), including ethylene homopolymers and copolymers. Preferably, the non-cling layer comprises at least 80 wt. % of LDPE, more preferably at least 90 wt. %, e.g. 95 wt. % or more, and most preferably the non-cling layer consists essentially entirely of LDPE. By xe2x80x9cconsists essentially entirely ofxe2x80x9d we mean that the non-cling layer may contain materials other than LDPE, for instance additives or other ethylene or xcex1-olefin polymers, but in insignificant amounts so as not to have any significant deleterious effect on the absence of cling from the non-cling side of the film, or the low noise level generated on unwinding of the film. Best results are obtained with a film having a 100 wt. % LDPE non-cling layer.
The density of the LDPE is in the range 0.915 to 0.940 g/cm3, and is preferably in the range 0.920 to 0.932 g/cm3, and is more preferably in the range 0.924 to 0.932 g/cm3, and most preferably 0.925 to 0.930 g/cm3. The melt index (MI) of this polyethylene is preferably at least 1 g/10 min., for ease of processing and stretching, and is more preferably in the range 1.5 to 5.5 g/lo min., and most preferably in the range 3 to 4.5 g/10 min., as measured by ASTM norm D1238 (190xc2x0 C./2.16 kg).
The non-cling layer has substantially no cling to a layer of itself. By this we mean that the non-cling layer exhibits a cling of at most 12 g/cm (30 g/inch), preferably at most 4 g/cm (10 g/inch), as measured following ASTM D4649-A3 on an unstretched film. Generally, the coefficient of friction of the non-cling later is below 1, and is typically in the range 0.4 to 0.9, preferably 0.45 to 0.8. This gives the non-cling layer a slippery feel, which is particularly desirable where products wrapped with the film may touch or slide against each other, and also helps to achieve a regular and moderate unwinding force from a supply roll, which contributes to a reduction in the noise generated on unwinding.
The level of cling between the cling and non-cling layers of the film of the present invention is typically in the range 39 to 236 g/cm (100 to 600 g/inch), preferably in the range 59 to 197 g/cm (150 to 500 g/inch), and more preferably in the range 79 to 118 g/cm (200 to 300 g/inch), as measured following ASTM D4649-A3 on an unstretched film.
As mentioned above, when the multilayer film is to include at least one layer in addition to the cling and non-cling layers, the properties of such at least one layer are selected according to the intended application of the film. Suitable examples of materials which may be utilised as an additional, core, layer include low density polyethylene (LDPE), having a density in the range 0.916 to 0.929 g/cm3, medium density polyethylene (MDPE), having a density in the range 0.93 to 0.945 g/cm3, high density polyethylene (HDPE), having a density greater than 0.945 g/cm3, and substantially linear polyethylenes, for instance linear low density polyethylene (LLDPE), having a density in the range 0.916 to 0.929 g/cm3, including homogeneously-branched LLDPE, and very low density polyethylene (VLDPE) having a density of less than 0.915 g/cm3. LLDPE is preferred.
It is preferred that the film of the invention is substantially entirely composed of polyethylene, including ethylene xcex1-olefin copolymers, to facilitate recycling, typically for use in a core or structural layer of another film. By xe2x80x9csubstantially entirelyxe2x80x9d typically we mean that the film of the invention is composed of at least 90 wt. % polyethylene, and preferably at least 95 wt.i polyethylene, and most preferably about 100 wt. % polyethylene.
Films in accordance with the present invention have low noise on unwinding from a supply roll, and therefore the environment in which they are used need not be subject to limited exposure times in order to meet hearing safety requirements. In general, noise levels on unwinding do not exceed around 85 dB, and are typically around 78 to 82 dB or less, for instance in the range of 75 to 80 dB, as measured using a RION NA-14 Sound Level Meter positioned at 30 cm from the unwinding point of a film supply roll, at an unwind speed of 15 m/min, and in an environmental noise level of 65 dB.
Due to the inherent cling and non-cling properties of the individual layers of the film of the invention, it is not necessary to include in those layers additives such as tackifiers, and slip and antiblock agents, to impart cling or non-cling characteristics. Preferably, therefore, the film is essentially free of such additives, thereby avoiding the problems commonly associated therewith. By xe2x80x9cessentially freexe2x80x9d of additives we mean that the amount of such additives, if present at all, is such that the cling or non-cling properties of the film are not appreciably changed. For instance, depending on the nature of the film, cling and/or anti-cling additives may be present in an amount of less than 500 ppm each, and typically considerably less than this, for instance less than 100 ppm or less than 50 ppm.
It may, however, be desirable to include in the film of the invention additives which change other properties of the film, for instance antioxidants, colorants, and other processing aids.
The multilayer film of the invention typically has a thickness of up to 150 xcexcm, preferably 5 to 100 xcexcm, and more preferably 9 to 50 xcexcm. Typically the non-cling layer will be thicker than the cling layer, the non-cling layer typically comprising up to 20%, preferably 8 to 12%, of the total thickness of the film, and the cling layer typically comprising up to 15%, preferably 5 to 9%, of the total thickness of the film. The thickness of the core layer, if present, is typically greater than each of the cling and non-cling layers, and generally comprises at least 65%, preferably 79 to 87%, of the total thickness of the film.
The multilayer film of the invention is a blown film. Suitable blown film processes are described in, for example, The Encyclopaedia of Chemical Technology, Kirk Othmer, 3rd Edition, (1981)16:416-417 and 18:191-192.
The multilayer film of the present invention is particularly suitable for use in load palletisation. This process generally involves threading a film from a supply roll through an automated or manual wrapping device, and then wrapping the film about an article or group of articles supported on a pallet, and sealing the film to itself. In the final, wrapped pallet, the non-cling layer will normally face outwards. This is advantageous because if the pallet comes into contact with another, similarly-wrapped pallet, which tends to occur during transportation, their outermost surfaces will slide past one another, and will not cling to one another, thereby avoiding damage to their wrappings.